1. Field of the Invention
The present invention relates to an electric focusing device known as a power focus device which drives a motor by manually operating a switch so as to perform a focusing operation of a picture-taking lens.
2. Related Background Art
A conventional method for focusing a picture-taking lens moves the lens by mechanically converting a rotational movement of a circular focusing ring to a linear movement by a helicoid mechanism or a cam mechanism, and therefore, gives an operator a good feeling of manipulating a camera.
Recently, auto-focusing cameras in which a picture-taking lens is focused by driving a motor have become quite popular. It has become increasingly common to provide a so-called power focus mechanism which moves a group of focusing lenses by turning on and off an electrical switch in accordance with the operation of an operation member to be manually operated, such as a push button, a seesaw lever and so on, and by controlling the rotation of a motor in response to a switch signal.
However, since a structure of the switch and the manual operation member in such a power focus mechanism is different from that of a circular focusing ring in a conventional mechanical focusing device, as good a feeling as that of the conventional focusing device cannot be obtained and a fine adjustment of the focusing is quite difficult.
In order to cope with this situation, a known method detects the amount of the rotation of an operation member, for example, a rotatable dial or ring, by a rotary encoder or the like and controls a motor in accordance with the detected amount of the rotation. Though this method can obtain a good operation feeling, it is advantageous in regard to cost and production because a mechanism for detecting the rotation is complicated and must be precisely manufactured, and because software for controlling the motor is also complicated.
U.S. Pat. No. 4,508,443 proposes, in a power focusing device using a push button which is easy to produce, a method of detecting the operation time of the switch, controlling a motor in accordance with the operation time of the switch, driving the motor while the switch is being operated if the operation time is long, and controlling the drive amount of the motor at a constant amount if the operation time is shorter than a predetermined time. Though this method can perform a fine adjustment of the focusing comparatively easily, because it is necessary to intermittently operate the operation member many times during the focusing operation in a fine adjustment state, it is difficult to achieve a quick focusing operation.